A recent development in magnetic resonance imaging (MRI) is functional magnetic resonance imaging (fMRI). fMRI relies on the detection of localized changes in signal intensity in T2*-weighted (or T2-weighted) images. For example, MRI may non-invasively map human cortical function without the use of exogenous contrast agents by relying on the blood oxygenation level dependent (BOLD) contrast due to the ability of deoxyhemoglobin to act as an endogenous paramagnetic contrast agent. Therefore, changes in the local concentration of deoxyhemoglobin within the brain lead to alterations in the magnetic resonance signal. Regional neuronal activation within the cerebral cortex leads to an increase in blood flow without a commensurate increase in oxygen extraction. Consequently, the capillary and venous deoxyhemoglobin concentrations decrease, leading to a localized increase in T2* and T2. This increase is reflected as an elevation of intensity in T2*-weighted and T2-weighted MR images. With its high contrast, T2*-weighted imaging is the predominant technique currently employed. It was initially applied to delineate the activity in the human visual cortex, motor cortex, and areas in the frontal cortex during speech.
Although application of fMRI in delineating cortical activity in the last few years has been remarkable, ideally, only intensity changes related to neuronal activation should be detected. In practice, many other sources contribute to image-to-image intensity fluctuation, leading to artifacts in the resultant functional maps. Thus, motion artifacts continue to be a major hindrance to the accurate detection of neuronal activity.
Sources of motion artifacts include system instability, subject movement, as well as normal physiological motions. Artifacts due to gross subject motion have been recognized as one possible source of false activation, and spatial registration of images can be beneficial in reducing these artifacts. Gross involuntary subject motion can be minimized by physically constraining the subject. Other physiology related motions, such as brain motion, respiration, and cardiac pulsation, are more difficult to compensate for and cannot be eliminated in a straightforward manner. These types of motion and their effects will be referred to as physiological fluctuation or physiological effect.